San Juan River Recovery Implementation Program Summary Report 1991-97

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11 slightly lower flows during August 1994 or may have been related to an overall decline of backwater habitat availability during recent years as noted by Bliesner and Lamarra (1996). Buntjer et al. (1994) indirectly evaluated availability of low velocity habitat types as part of the Early Life History Fisheries Survey of the San Juan River during 1993. Since backwaters were the target habitat in their investigation, analyses were based on the differences in amount of backwater habitats ' sampled during different periods and flows. Backwater habitats accounted for 53 percent of the total area seined during spring 1993 at flows of 5,860 to 6,700 cfs. During July, at flows of 740 to 1,200 cfs, 84 percent of the total area seined was backwater habitat, and in early August at flows of 412 to 533 cfs, 97 percent of the area seined was backwater habitat. By September, at flows of 1,180 to 2,390 cfs, 68 percent of the area seined was backwater habitat. These data, although somewhat qualitative, suggest that available backwater habitat increased with decreasing discharge in the San Juan River. Summary Habitat availability at different flow levels is relatively well understood for the San Juan River. The river is generally run- dominated, especially at high flows, and as flows are reduced lower velocity habitats increase in abundance but still represent only a small percentage of the total habitat available. Although additional fine tuning by collection of additional data will continue in 1997 and 1998, for the most part this objective has been completed. 5.2.4. Model Flow and Habitat Relationships. Contributing Investigations C 9 9 Studies by Bliesner and Lamarra (1993, 1994, 1995) investigated development of a model for flow and habitat relationships. Findings Conceptually, habitat modeling took two tracks: (1) hydraulic modeling of key features was used to predict formation of spawning and backwater habitats, and (2) empirical modeling was used to develop relationships between habitat and flowrate. In 1992 channel geometry in representative secondary channels was measured with the intent of utilizing the HEC -2 water surface profile model to determine the flow at which secondary channels begin to flow. Model testing was completed to determine data requirements and assess predictability (Bliesner and Lamarra 1993; R. Bliesner 1995, pers. comm.). Since some secondary channels form backwaters at lower stages, HEC -2 modeling will be used to assess hydraulic conditions necessary to maintain these habitats (R. Bliesner 1996, pers. comm.). HEC -2 modeling and sediment transport analysis are being used to study spawning habitat development and maintenance. Spawning sites were characterized in terms of channel geometry and substrate characterization. The characteristics found at known spawning sites are used to identify other areas that would be suitable for spawning. HEC -2 models of each of these sites will San Juan River Recovery Implementation Program December 1997 Summary Report 16 PR -602 -1